1. Technical Field
The invention relates to a driving apparatus for writing an image onto a liquid crystal device that displays and records the image using a liquid crystal and a photoconductor, and a method for driving the liquid crystal device.
2. Related Art
Various researches have been made on a rewritable marking technique that is highly convenient. As one direction of the rewritable marking technique, a display medium using a cholesteric liquid crystal has gained public attention recently. This is because the display medium using the cholesteric liquid crystal has characteristics of having a memory property capable of holding display without power source, providing a bright display due to non-use of a polarizer, and allowing color display without using a color filter.
A planer state of the cholesteric liquid crystal (chiral nematic liquid crystal) causes a selective reflection phenomenon in which light incident in parallel to a helical axis is divided into a right-handed polarized light and a left-handed polarized light, circularly polarized light component, which coincides with a twist direction of spiral, is Bragg reflected, and the remaining light are transmitted. The central wavelength λ and the reflection wavelength width Δλ of the reflected light are represented as λ=n·p, Δλ=Δn·p, where p denotes a helical pitch, n denotes an average refractive index within a plane orthogonal to the helical axis, and Δn denotes a birefringence index. The reflected light from the cholesteric liquid crystal layer of the planer state displays a vivid color dependent on the helical pitch.
The cholesteric liquid crystal with positive dielectric anisotropy has three states, that is, the planar state, the focal conic state and the homeotropic state. In the planar state, the helical axis is perpendicular to the cell surface, and the selective reflection phenomenon occurs with respect to the incident light as shown in FIG. 17A. In the focal conic state, the helical axis is almost parallel to the cell surface, and the incident light is slightly forward-scattered and transmitted as shown in FIG. 17B. In the homeotropic state, the helical structure is relaxed, and a liquid crystal director is oriented in the electric field direction to fully transmit the incident light as shown in FIG. 17C.
Of the three states, the planer state and the focal conic state can exist bi-stably without electric field. Accordingly, the state of the cholesteric liquid crystal is not uniquely determined for a strength of an electric field applied to the liquid crystal layer. If the planer state is in the initial state, the cholesteric liquid crystal changes in order of the planer state, the focal conic state and the homeotropic state with increase in strength of the electric field. If the focal conic state is in the initial state, the cholesteric liquid crystal changes in order of the focal conic state and the homeotropic state with increase in strength of the electric field.
On the other hand, when the strength of the electric field applied to the liquid crystal layer is rapidly decreased to zero, the planer state and the focal conic state keep the as-is state, and the homeotropic state changes to the planer state.
Accordingly, immediately after a pulse signal is applied, the cholesteric liquid crystal layer shows a switching behavior shown in FIG. 18. When the applied voltage of the pulse signal is greater than or equal to Vfh, the cholesteric liquid crystal layer changes from the homeotropic state to the planer state and becomes the selective reflection state. When the applied voltage is between Vpf and Vfh, the cholesteric liquid crystal layer becomes a transmission state due to the focal conic state. When the applied voltage is less than or equal to Vpf, the cholesteric liquid crystal layer continuously keeps a state that is the state before the pulse signal is applied, that is, keeps the selective reflection state due to the planer state or keeps the transmission state due to the focal conic state.
In FIG. 18, the vertical axis represents a normalized light reflectivity, in which the light reflectivity is normalized, supposing that the maximum light reflectivity is 100 and the minimum light reflectivity is 0. A transition area exists between any two of the planer state, the focal conic state and the homeotropic state. Therefore, the selective reflection state is defined as a state where the normalized light reflectivity is 50 or more, and the transmission state is defined as a state where the normalized light reflectivity is less than 50. Also, Vpf denotes the threshold voltage for the state change between the planer state and the focal conic state, and Vfh denotes the threshold voltage for the state change between the focal conic state and the homeotropic state.
The display medium with the cholesteric liquid crystal may have a structure in which the liquid crystal is sealed in as the continuum phase between one pair of display substrates, a PDLC (Polymer Dispersed Liquid Crystal) structure in which the cholesteric liquid crystal is dispersed like drops in the polymer binder, and a PDMLC (Polymer Dispersed Microencapsulated Liquid Crystal) structure in which the microencapsulated cholesteric liquid crystal is dispersed in the polymer binder (for example, see JP Hei. 7-9512 B (corresponding to U.S. Pat. No. 4,435,047), JP Hei.9-236791 A (corresponding to U.S. Pat. No. 6,067,135), Japanese Patent No. 3178530).
Using the PDLC structure or PDMLC structure suppresses the fluidity of liquid crystal. Therefore, distortion of an image due to a bend or pressure is reduced. Thereby, the flexible medium can be realized. Also, the color display can be made by directly laminating plural cholesteric liquid crystal layers thereon, or a display medium for addressing an image with an optical signal can be made by laminating a photoconductor layer thereon. Furthermore, a display layer can be formed using a thick-film print technology. Thereby, there is such an advantage that the manufacturing method is simplified and the cost is reduced.
Many display media using this technique have been proposed (for example, see JP Hei. 11-237644 A).
The photo-writing type (photo-address type) display medium according to this technique performs monochrome display in various hues having a memory property under no electric field or performs color display having a memory property under no electric field, by switching between (A) the selective reflection state provide by the planer state and (B) the transmission state provided by the focal conic state, using a bi-stable phenomenon of the cholesteric liquid crystal.
The liquid crystal device according to this technique can form an image on the entire surface without simultaneous exposure. Therefore, an image can be written by scanning the surface of the liquid crystal device using a scanning-type exposure device, for example, a laser beam exposure device or a light emitting diode array.
FIG. 19 is a schematic view schematically showing how to write an image on the liquid crystal device with the scanning-type exposure device according to this technique. The liquid crystal device according to this technique has a pair of electrode substrates, a display layer that is the liquid crystal layer, an OPC layer that is the photoconductor layer, and a light shielding layer as shown in FIG. 19 The display layer and the OPC layer are disposed between the pair of electrode substrates. Also, the display layer and the OPC layer are laminated with sandwiching the light shielding layer therebetween. After resetting the entire surface of the display layer to the planer state, a desired recording image can be written by scanning and imagewisely exposing the surface of the OPC layer using the exposure device such as a line head or a beam scanner while a predetermined bias voltage is being applied to the both transparent electrodes.
JP 2007-17461 A (corresponding to U.S. 2007/0008262 A) has proposed a driving method in which a write operation is divided into an initialization step, a write step and a display determination step. This driving method does adopts a state change from the focal conic state to the homeotropic state at a higher state change speed as a switch in the write step, but does not adopt a state change from the planer state to the focal conic state at a slower state change speed. In this driving method, a write time is greatly shortened by time sharing only an operation in the write step at the higher state change speed and scanning the entire surface of the liquid crystal device for every pixel or every line.